One of the many nice things about later 235/261 blocks is that there is a large hole in the block when you remove the water pump. I used thick copper wire to clear out a 261 block - it took a lot of effort and a lot of time. If you cannot use a copper wire to enter the block through the drain cock near the driver's side rear of the block, you've got problems. Coolant water must be able to circulate around the rear of the block. Keep at it.
